Disable dhcp_domain distribution when dhcp_domain is empty

fixes bug 1099625

Change-Id: Ib86770345c46a0dd8bb38a4c3e435420170828af
Shorten the DHCP default resync_interval

Fixes bug 1128180

Change-Id: Ie63ef674b5e05fab659e675774a1b25cd57d4c41
Merge "Disable dhcp_domain distribution when dhcp_domain is empty" into stable/folsom
Merge "Shorten the DHCP default resync_interval" into stable/folsom